This invention relates to a gas tracer composition and method. More specifically, this invention relates to methods for the tagging of methane and to tagged methane compositions. The term "methane" is used herein to denote not only pure methane, but also to natural gas compositions which are mainly methane but may contain minor proportions of other gases, for example helium, ethane and hydrogen sulfide. The term "methane" as used herein also extends to so-called synthetic natural gas i.e. methane produced by chemical synthesis and intended to be used as a replacement for natural gas. "Methane" may also refer to liquified natural gas (LNG) as it is produced either domestically or overseas. LNG is made by the pressurized and cooling liquifaction process as it is performed on natural gas coming out of the ground.
The demand for natural gas is of course largely seasonal, because much of the natural gas is consumed for heating purposes. Since it is desirable to maintain a steady output from gas wells throughout the year, it is necessary to store enormous quantities of natural gas during the warmer months of the year in order to meet the peak winter demands, thereby to allow proper matching of the relatively steady production of natural gas against the fluctuating demand therefor. Because of the enormous quantities of gas (of the orders of billions of standard cubic feet) which have to be stored, the construction of artificial storage facilities for the gas is economically unattractive, and most of the gas is thus stored in natural formations, primarily exhausted natural gas production fields, though salt domes may also be used.
The geology of the exhausted gas fields used for storage may be both complex and not entirely known; for example, unknown to the operators there might be underground communication between an exhausted gas field used for storage and a nearby field which is still being used for production. Furthermore, there is a possibility that someone else drilling in the vicinity may penetrate the gascontaining formations during drilling operations, and proceed to produce gas from the storage field, thereby (knowingly or unknowingly) stealing gas from its rightful owner. Accordingly, it is very desirable that the owner of the gas being stored in a gas storage field be able to "tag" the gas by adding a minor proportion of a tracer compound thereto, so that should the gas placed in the storage field migrate to a nearby field or be removed by unauthorized persons it can be identified; it will be appreciated that generally natural gas is a fungible product and, absent the addition of some tracer compound thereto, it is not possible to determine the source of any particular sample of natural gas.
Hitherto, the main tracer compound used to tag methane has been ethylene. Until recently, ethylene was thought to be a good tracer compound because it moves through a gas field in a manner very similar to methane (and thus does not become separated from the methane as the methane diffuses through large gas fields) and because it was thought not to occur in natural methane supplies. Furthermore, even very small amounts of ethylene are easily detectable in methane. Unfortunately, it has now been discovered that although ethylene does not occur naturally as a contaminant of methane, there are at least two ways in which methane may be contaminated with traces of ethylene even though it has not been deliberately tagged wtih ethylene. Firstly, some fields now used for methane storage have previously been used for storing coke-oven gas, which contains considerable amounts of ethylene. The storage of coke-oven gas in these fields has left a residue of ethylene within the fields, so that methane supplies pumped into such fields do become contaminated with this residual ethylene. Secondly, some of the processes by which natural gas fields are prepared for use as either storage or production facilities produce ethylene; in particular, treatment of gas wells with acid to clean and open casing perforation results in contact of the acid with the steel in the well pipe, thus producing trace quantities of ethylene. Accordingly, ethylene is no longer useful as a reliable tracer compound, and there is thus a need for other tracer compounds for tagging methane.
The selection of appropriate tracer compounds for use in tagging methane supplies presents very considerable difficulties, in view of the very exacting requirements which a tracer compound must meet. Firstly, the tracer compound must be one which does not occur even in minute quantities in natural supplies of methane, which may contain a variety of contaminants including the inert gases, hydrogen sulfide, ethane, propane and other hydrocarbons. In addition, the tracer compound must diffuse through a gas storage field in a manner very similar to that of methane, otherwise if a single injector well or a small cluster of injector wells are used to inject gas into a large storage field, the tracer may remain in the vicinity of the injector well(s), leaving gas at large distances from the injector wells effectively untagged. Since many gas storage fields contain traces of liquid hydrocarbons, the tracer compound must not be too soluble in such hydrocarbons, otherwise the tracer will be absorbed by the liquid hydrocarbons, again leaving the gas stored in the field untagged. The tracer compound must be stable for periods of at least several months in the presence of methane and any possible impurities in natural methane supplies. Moreover, in view of the enormous volumes of gas which have to be tagged (a single natural gas field may typically store ten to fifty billion standard cubic feet (Bscf) of methane) and the difficulties which may be encountered in transporting large quantities of tracer compounds to the gas storage fields, economic considerations dictate that any tracer compound be detectable in concentrations of no more than a few parts per million in methane. Since investigations of unauthorized removal of tagged natural gas from a storage field may require analysis of the suspect gas in the field, the tracer compound must be detectable at such very low concentrations in methane using readily portable apparatus. Finally, since it is not practicable to remove the tracer compound from the methane after it has been withdrawn from storage, the tracer compound must not interfere with or create dangers during any of the normal uses of methane.
It should be noted that the provision of tracer compounds for methane stored in gas storage fields involves very different problems from the provision of tracer compounds which are used to detect leaks in gas pipelines. When it is desired to use a tracer compound to find the position of a leak in a gas pipeline, only a relatively small amount of gas flowing through the pipeline has to be tagged, and thus it is practical to use much higher concentrations of tracer compounds. Indeed, since the leaking pipeline is frequently shut down, it may be possible to fill the pipeline with the "tracer" compound in order to produce the maximum possible concentration of tracer compound adjacent the leak. Thus, many techniques for detecting leaks in pipelines are totally inapplicable to tagging gas stored in storage fields. Furthermore, tracer compounds used for detecting leaks in pipelines do not have to meet the requirements of (1) having a mobility through rock, (2) not dissolving in hydrocarbons and (3) being stable for months at a time, which a tracer compound intended for use in gas fields must meet. For this reason, most of the tracer compounds suggested for use in detecting leaks from pipelines are totally unsuitable for use in tagging methane to be stored in gas storage fields. For example, U.S. Pat. No. 3,523,771, granted Aug. 11, 1972, to Anderson, describes a process for detecting leaks from pipelines in which there is added to a fuel gas passing through the pipeline from 0.01 to 10% by weight of certain organometallic compounds which react spontaneously with air to produce a smoke visible at the source of a leak in a pipeline. The use of these organometallic compounds as tracers in gas fields would be entirely inappropriate since the amount of tracer which would have to be added would be so large as to be economically impracticable, and in any case, gas recovered from a natural gas storage fields almost never comes into contact with air. Furthermore, it is most unlikely that such organometallic compounds would diffuse through a gas storage field at the same speed as methane, and, as described in U.S. Pat. No. 3,523,771, the addition of organometallic compounds to the field gases does present serious problems in later using the fuel gases unless that organometallic compounds are removed; while it is practicable to remove the organometallic compounds from the relatively limited amount of gas which has to be tagged for leak detection purposes, it is utterly impractical to attempt to remove a trace of compound from the huge quantities of gas recovered from a gas storage field. Accordingly, although the process described in U.S. Pat. No. 3,523,771 may be a useful method of tagging fuel gases to detect leaks in pipelines, it is entirely impractical as a method of tagging gas to be stored in a gas storage field.
After extensive evaluation involving over 140 potential tracer compounds, we have now discovered two new tracer compounds for methane which meet all the aforementioned requirements.